Working In the Hot Aisle: Choosing a Design

Part 1 of this saga can be found here.

As mentioned in the previous post, there several ways to skin the airflow separation cat. Each has its advantages and disadvantages.

Initially we considered raised floor cooling. Dating back to the dawn of computing, raised floor datacenters consist of chillers that blow cold air into the space under the raised floor. Floor tiles are replaced with perforated tiles in front of or underneath the enclosures to allow cold air into the equipment. Hot air from the equipment is then returned to the A/C units above the floor. The raised floor also provides a hidden space to route cabling, although discipline is more difficult since the shame of poor cabling is hidden from view. While we liked the clean look of raised floors with the cables hidden away, the cost is high and the necessary entry ramp at the door would have taken up too much floor space in our smaller datacenter.

We also looked at a hot aisle design that uses the plenum space above a drop ceiling as the return. Separation is achieved with plastic panels above the enclosures, and the CRAC units are typically placed at one or both ends of the room. Because this was a two-row layout in a relatively tight space, it was difficult to find a location for the CRACs that would avoid creating hot spots.

The decision became a lot easier when we found out that one of the spaces we were vacating had APC in-row chillers that we could pick up at a steep discount. The in-row units are contained within a single standard rack enclosure, so they are ideal for a hot aisle/cold aisle configuration. They solved the hot spot issues, as they could be placed into both rows. They also use temperature probes integrated into the nearby cabinets to keep temperatures comfortable efficiently.

APC In-Row RD. APC makes a kit to extend the height to 48U if used with tall racks. (Photo by APC)

With the cooling situation sorted, we turned our attention to containment. We opted for the Schneider/APC EcoAisle system, which provided a roof and end-doors to our existing two-row enclosure layout to create a hot aisle and a cold aisle. The equipment fans pull in cooler air from the cold aisle and exhaust hot air into the hot aisle, while the in-row chillers pull hot air from the hot aisle and return chilled air back into the cold aisle.

There are two options for this configuration.  A central cold aisle can be used, with the rest of the exterior space used as a hot aisle.  This possibly could reduce energy consumption, as only the central aisle is cooled and therefore the room doesn’t need to be sealed as well from air leaks.

The second option, which we ended up choosing, was a central hot aisle.  In our case, the exterior cold aisles gave us more room to rack equipment, and using the entire room as the cold aisle gives us a much larger volume of cool air, meaning that in the case of cooling system failure, we have more time to shut down before temperatures become dangerous to the equipment.

The central hot aisle is covered by a roof consisting of lightweight translucent plastic insulating panels, which reduce heat loss while allowing light in. (The system includes integrated LED lighting as well.) The roof system is connected to the fire system, so in the case of activation of the fire alarm, electromagnets that hold the roof panels in place will release, causing the roof panels to fall in so the sprinklers can do their job.  We can also easily remove one or more panels to work in the cable channel above.

APC EcoAisle configuration. Ours is against the wall and has one exit door. (Photo APC).

Our final design consists of eleven equipment racks, three UPS racks and four chiller racks.  This leaves plenty of room for growth, and in the case of unexpected growth, an adjacent room, accessible by knocking out a wall, doubles the capacity.

We decided on 48U racks, both to increase the amount of equipment we can carry and to raise the EcoAisle’s roof. To accommodate networking, one enclosure in each aisle is a 750mm “network width” enclosure to provide extra room for cable management. As the in-row CRACs were only 42U high, Schneider provides a kit that bolts to the top of the units to add 6U to their height.

Next time:  Power and cooling

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